Eusauropleura digitata (originally Sauropleura, Cope 1868; Romer 1930; Carroll 1970; Late Carboniferous, 310 mya) nests as a late-surviving basalmost stem reptile in the LRT. The genesis for this genus was in the earliest Carboniferous based on the more derived Silvanerpeton (Viséan, 335 mya). A dense layer of belly scales (not shown), a larger manual digit 5, and a taller ilium, among other traits, distinguish this specimen from Gephyrostegus. A larger manus, ischium and giant caudal transverse processes (ribs) relative to the torso are unique traits among close relatives. Note the lack of ribs in the lumbar area, where eggs develop before they are layed. The eggs were relatively large based on the greater depth of the ischium.
First considered a microsaur
(Cope 1868), then a gephyrostegid (Romer 1930, 1950; Carroll 1970), Eusauropleura was identified as more primitive than Gephyrostegus, but still terrestrial, not aquatic and close to the ancestry of reptiles, but never reptiles. As determined here in 2011, there is no list of traditional reptile skeletal traits that determines the reptile status of Gephyrostegus. Only the nesting of Gephyrostegus as the last common ancestor of all reptiles in the the LRT tells us it was laying eggs with an amnion, the ONLY trait needed to determine its reptile status. Silvanerpeton was likely also a reptile because phylogenetic descendants of late-surviving Gephyrostegus are also found in coeval Viséan strata. And that makes the last common ancestor of Silvanerpeton and Gephyrostegus, Eusauropleura, also a late-surviving basalmost reptile. Earlier workers did not enter Eusauropleura, Silvanerpeton and Gephyrostegus into a wide gamut phylogenetic analysis and so did not recover a last common ancestor status for these taxa.
Another specimen attributed to Eusauropleura
AMNH 6860 (Moodie 1909, Carroll 1970), is a bit more jumbled, more incomplete and more difficult to reconstruct. The posterior process of the ilium is easy to see in this specimen and it provides attachment points for more than one sacral rib, which are difficult to determine in the scattered fossil.
Yet another specimen attributed to Eusauropleura
PU 16815 is an isolated pectoral girdle, bones lacking in the other specimens and therefore not comparable.
Scales
According to Carroll 1970, "Scales, both dorsal and ventral, are conspicuous in these specimens. The body was protected by heavy, oblong scales, overlapping to form a chevron pattern, between the pelvic and pectoral girdles. Were they not associated with the skeleton, they would be difficult to distinguish from those of [more primtive] embolomeres. Laterally the scales assume a more oval outline, become thinner, smaller and less extensively overlapping. The dorsal scales are small, thin and round. Where worn, all the scales exhibit a pattern of fine ridges, running parallel with the margins. These form a pattern of concentric ridges in the dorsal scales, similar to that of [more primitive] discosauriscids. Except for the heavier ossification of the dorsal scales, those of Eusauropleura are generally similar to those of Gephyrostegus."
Basal to Eusauropleura
are taxa close to the Reptilomorpha - Lepospondyli split. These include Eucritta and Utegenia (Fig. 2) all derived from Late Devonian Tulerpeton. In Eusauropleura the central elements are very thin-walled, forming little more than a husk around the large notochord. This affirms the primitive state of basalmost reptiles, derived from Devonian tulerpetids. |
